Acid-free pyrolytic synthesis of M-N-C catalyst

This application claims the benefit of U.S. provisional application Ser. No. 63/355,699 filed Jun. 27, 2022, the disclosure of which is hereby incorporated in its entirety by reference herein.

The invention was made with Government support under Contract No. CHE-1338173 awarded by the National Science Foundation Major Research Instrumentation Program and under Contract No. DMR-2011967 awarded by the National Science Foundation through the UC Irvine Materials Research Science and Engineering Center. The Government has certain rights to the invention.

In at least one aspect, the present invention is related to energy, electrocatalytic materials, and non-platinum group metal catalysts. In another aspect, the present invention is related to a rapid facile synthesis of a highly active electrocatalyst for fuel cell/energy conversion applications including but not limited to the oxygen reduction reaction, COelectroreduction, and Nreduction reaction.

Among the most active platinum group metal-free benchmark catalysts for energy conversion (e.g., fuel cell/COelectroreduction/Nelectroreduction) are metal-nitrogen-doped carbon electrocatalysts synthesized by various means. These metal-nitrogen-doped carbon catalysts are produced by pyrolytic means and offer atomically dispersed metal active sites with high atom utilization resulting in high electrocatalytic activity. The synthesis proceeds through one of three major categories of synthesis protocols: either through soft-template approach[1-5], hard-templating with silica[6-12], sacrificial polymer approach [13-15], or through a combination of the methods above such as by the combined sacrificial polymer and sacrificial metal organic framework (MOF) approach [15].

Use of MOFs such as Zn-based ZIF-8 as pyrolytic precursors is a powerful soft-templating approach to synthesizing M-N—C catalysts. Utilizing ZIF-8, an optimized Fe—N—C was synthesized by soft-template method in 2011 which achieved an impressive 0.9 W cm-2geo in an H2/O2 polymer electrolyte membrane fuel cell (PEMFC).[16] The silica hard-templating approach discussed more below has been refined for over a decade which has yielded arguably the top M-N—C candidate for replacing platinum-group metal electrocatalysts. A third method is the sacrificial polymer approach which utilizes nitrogen-doped polymers (e.g., polyaniline, and polypyrrole) as dual nitrogen/carbon source which upon polymerization in the presence of a metallic salt, and pyrolytic treatment followed by acid washing yields a highly active M-N—C catalyst[13]. These methods each offer different routes to a highly efficient electrocatalyst however they each also have substantial synthetic limitations which impede their large-scale commercialization.

These challenges can be simplified to three main obstacles: (i). use of the hard-template approach or the sacrificial polymer approach requires an acid-wash step (usually in harsh hydrofluoric acid or caustic KOH) which increases the synthesis time substantially and requires the disposal of large quantities of acids/bases which increases the overall cost of the process, (ii). the syntheses produce catalysts with modest active site density (SD) which impedes their overall catalytic throughput, and (iii). the intermediate acid-wash step to produce the state-of-the-art M-N—C catalysts before the final pyrolysis complicates the process by requiring a hands-on transferring of the step for acid-wash and back to the pyrolysis chamber which limits its industrial scale-up.

By way of example, the state-of-the-art catalyst identified by Chen et al [6] this year and [17] in 2020 are produced by hard-templating with silica. This method was developed by the Atanassov lab in 2008 and has since been commercialized by Pajarito Powder© due to the high electrocatalytic activity of the catalysts produced by this synthesis technique. This approach initially necessitated a metal macrocycle (e.g., metal porphyrin), but the process has since been simplified to utilize simple metallic salts, and common nitrogen/carbon precursors mixed in optimum ratios with silica particles of varying particle size/surface areas[19, 20]. The proper ratio of silica precursor(s) and their respective particle sizes have a substantial impact on the final pore size distribution and eventually lead to a hierarchical porosity with beneficial electrocatalytic properties. This precursor mixture is subjected to a first pyrolysis in inert or reductive atmosphere(s) followed by an acid washing step in highly concentrated (25 wt. %) hydrofluoric acid (HF) to generate a high degree of mesoporosity through the hard templating effect. The silica templating production of M-N—C catalysts by Pajarito Powder has been shown to produce less M-Nx (a chemical moiety integral to its electrocatalytic activity) than laboratory batches from University of New Mexico (UNM) where the technique was developed[17]. This is evidence of the limitations in the industrial scale-up of this synthesis technique. To date, no silica hard-template synthesis of M-N—C catalysts has been accomplished that completely avoids the utilization of harsh acids/bases while also not relying on the utilization of metal macrocycles.

Accordingly, there is a need for improved methods for making M-N—C catalysts.

In at least one aspect, a method for making a catalyst is provided. The method includes a step of forming a first reaction mixture that includes a metal source, a nitrogen source, and at least one silica template. Characteristically, the at least one silica template includes silica particles. A combination of the first reaction mixture and a fluorinated polymer is mechanochemically mixed (e.g., ball milled) to form a first pre-pyrolysis powder. A first pyrolysis of the first pre-pyrolysis powder is performed at a first temperature greater than about 800° C. under an inert atmosphere to form a first pyrolyzed composition. Advantageously, at least a portion of silica particles is removed and mesostructured carbon is formed. The first pyrolyzed composition is mechanochemically mixed (e.g., ball milled) to form a second pre-pyrolysis powder. A second pyrolysis of the second pre-pyrolysis powder is performed under a reductive atmosphere at a second temperature that is greater than about 800° C. to form a final catalyst powder.

In another aspect, the catalysts provided herein present selectivity that exceeds the state-of-the-art single Ni site catalysts for COreduction to CO prepared by more time-consuming methods or through resorting to pyrolysis of metal porphyrins.[22-6] The catalysts present Ni nanoparticiesimetallic Ni phase(s) which are separate from the exclusively graphitic/amorphous carbon phases observed by XRD/HRTEM for single Ni sites in Ni—N—C(atomically dispersed Ni—N—C). These sites when exposed to aqueous electrolyte usually promote the parasitic production of 112 and hence are usually avoided in the synthesis of single-atom M-N—C catalysts.[23, 27, 28] In the catalyst produced by the method set forth herein, the presence of Nickel nanoparticles does not promote HER and the catalyst remains selective for CO at all potentials. Additionally, the activity as measured by electrolysis on a benchtop flow cell is comparable or higher than the state-of-the-art catalyst produced by competing syntheses. [29, 30] Implementation into an industrial-type electrolyzer is needed to assess their overall activity and stability at a high current density similar to the investigation by Moller et al. However, the results show that the catalyst shows high activity and selectivity on a benchtop flow cell which merits its industrial consideration for a COreduction cathode materials

In another aspect, the catalysts described herein can be used for the reduction of COto CO.[31]

In another aspect, a fluorinated polymer is used for the removal of silica during a first pyrolysis step.

In another aspect, the catalysts described herein can be used for ORR applications in fuel cells or OER in alkaline electrolyzers. It effectively may be used to update the sacrificial support method and sacrificial polymer approach which both require an acid/alkaline wash step [32].

In another aspect, a method for making a nitrogen-doped porous carbon is provided. The method includes a step of forming a first reaction mixture that includes a nitrogen and carbon source, a silica source, and a fluorinated polymer. The first reaction mixture is mechanochemically mixed to form a first pre-pyrolysis powder. A first pyrolysis of the first pre-pyrolysis powder is performed to form a first pyrolyzed composition. The first pyrolysis is performed at a first temperature greater than about 800° C. and under an inert atmosphere such that at least a portion of silica particles is removed and mesostructured carbon is formed. A second pyrolysis of the first pyrolyzed composition is performed to form a nitrogen-doped porous carbon. Typically, the second pyrolysis can be performed under a reductive atmosphere at a second temperature that is greater than about 800° C.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary the term “polymer” includes “oligomer,” “copolymer,” “terpolymer,” and the like; molecular weights provided for any polymers refers to weight average molecular weight unless otherwise indicated; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.

It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.

The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps.

The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.

It should also be appreciated that integer ranges explicitly include all intervening integers. For example, the integer range 1-10 explicitly includes 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Similarly, the range 1 to 100 includes 1, 2, 3, 4 . . . 97, 98, 99, 100. Similarly, when any range is called for, intervening numbers that are increments of the difference between the upper limit and the lower limit divided by 10 can be taken as alternative upper or lower limits. For example, if the range is 1.1. to 2.1 the following numbers 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2.0 can be selected as lower or upper limits. In the specific examples set forth herein, concentrations, temperature, and reaction conditions (e.g. pressure, pH, etc.) can be practiced with plus or minus 50 percent of the values indicated rounded to three significant figures. In a refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, etc.) can be practiced with plus or minus 30 percent of the values indicated rounded to three significant figures of the value provided in the examples. In another refinement, concentrations, temperature, and reaction conditions (e.g., pH, etc.) can be practiced with plus or minus 10 percent of the values indicated rounded to three significant figures of the value provided in the examples.

In the examples set forth herein, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 50 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In a refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 30 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In another refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 10 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples.

Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.

The term “M-N—C catalyst” refers to a catalyst in which a transition metal is coordinated with nitrogen and carbon atoms. M-N—C catalyst car used in cross-coupling reactions, hydrogenation reactions, oxidation reactions, and nitrogen fixation.

Abbreviations:

In an embodiment, a method for making a catalyst is provided. The method includes a step of forming a first reaction mixture that includes a metal source, a nitrogen source, and at least one silica template. Characteristically, the at least one silica template includes silica particles. A combination of the first reaction mixture and a fluorinated polymer is mechanochemically mixed (e.g., ball milled) to form a first pre-pyrolysis powder. A first pyrolysis of the first pre-pyrolysis powder is performed at a first temperature greater than about 800° C. under an inert atmosphere (e.g., a 95% to 100% nitrogen gas environment) to form a first pyrolyzed composition. Advantageously, at least a portion of silica particles is removed to form micron sized and/or mesostructured (e.g., 2 to 50 nm) carbon. The first pyrolyzed composition is mechanochemically mixed (e.g., ball milled) to form a second pre-pyrolysis powder. A second pyrolysis of the second pre-pyrolysis powder or the first pyrolyzed composition is performed under a reductive atmosphere at a second temperature that is greater than about 800° C. to form a final catalyst powder. In one variation, the catalyst is a transition metal-nitrogen-carbon catalyst. In some variations, the first temperature and the second temperature are greater than about, in increasing order of preference, 800° C., 850° C., 900° C., or 1000° C. and less than, in increasing order of preference, 1800° C., 1600° C., 1500° C., 1400° C., or 1300° C.

Examples of mechanochemical mixing, include, but are not limited to, high energy ball mixing, mortar and pestle grinding, vibrational ball milling, twin screw extrusion, planetary ball milling, stirred media milling, and the like. In a refinement, the first reaction mixture is homogenized prior to the first mechanochemically mixed step.

In another refinement, the reductive atmosphere for the second pyrolysis includes at least 5 weight percent ammonia in nitrogen gas. In some refinements, the reductive atmosphere for the second pyrolysis includes at least in increasing order of preference, 5 weight percent, 10 weight percent, 15 weight percent, 20 weight percent, or 25 weight percent, ammonia in nitrogen gas, and at most in increasing order of preference 40 weight percent, 35 weight percent, 30 weight percent, 28 weight percent, or 26 weight percent, ammonia in nitrogen gas

In a variation, for the first pyrolysis, a first initial temperature is ramped up to a first final temperature over a first predetermined time period, the first final temperature being held for a first hold time. In a refinement, the first initial temperature is ramped up with a rate of about 1 to 10° C./minute, the first final temperature is from about 800 to 1000° C., and the first hold time is 1 to 5 hours.

In a variation for the second pyrolysis, a second initial temperature is ramped up to a second final temperature over a second predetermined time period, the second final temperature being held for a second hold time. In a refinement, the second initial temperature is ramped up with a rate of about 1 to 10° C./minute, the second final temperature is from about 800 to 1000° C., and the second hold time is 0.1 to 3 hours.

As set forth above, a fluorinated polymer is used for removal of silica during the first pyrolysis step. In a refinement, the fluorinated polymer is a perfluorinated polymer. Examples of fluorinated polymers include poly(tetrafluoroethylene) (i.e., TEFLON®), fluorinated ethylene propylene, perfluoroethylene propylene, and the like.

As set forth above, the first reaction mixture can include a metal source. Typically, the metal source is a metal salt, and in particular, metal nitrates, metal sulfates, metal carbonates, and the like. Examples of metal sources include but are not limited to an iron(III) salt, a nickel(II) salt, a manganese(II) salt, a cobalt(II) salt, and combinations thereof.

In a variation, the nitrogen source includes amide functional groups or amino functional groups. For example, compounds having amide functional groups can be described by formula R—(C═O)—NRRwherein R, R, Rare each independently H or Calkyl where at least one of R, R, Rare a Calkyl. Compounds having amino functional groups NRRRwherein R, R, Rare each independently H or Calkyl where at least one of R, R, Rare a Calkyl. In a refinement, the nitrogen source includes nicarbazin. Nicarbazin is a combination of, and in particular, an equimolar complex of 1,3-bis (4-nitrophenyl) urea and 4-6 dimethyl-2-(1H)— pyrimidinone. It should also be appreciated that these nitrogen sources can also function as carbon sources.

In a variation, the at least one silica template includes fumed silica or precipitated silica. In a refinement, the at least one silica template includes silica having a particle size distribution that induces a hierarchical porosity in the final catalyst powder. In a further refinement, the at least one silica template includes silica particles having a diameter less than about 500 nm. In some refinements, the at least one silica template includes Stober spheres.

In another embodiment, an M-N—C catalyst formed by the methods set forth above is provided. In such catalysts, the nitrogen atoms are introduced into the carbon support to modify and enhance the catalytic properties of the metal. Advantageously, the M-N—C catalysts can be used to catalyze various electrochemical reactions. Such reactions find applications in energy conversion and storage. In a variation, the average pore diameters are from about 5 to 100 nm and the average BET surface area is from about 400 to 1000 mg.

In another embodiment, a supported catalyst includes a precious group metal catalyst supported on the catalysts, and in particular, the M-N—C catalysts set forth above. Examples of precious metals include gold, silver, platinum, palladium, rhodium, and ruthenium.

Referring to, a flowchart depicting a method for forming nitrogen-doped porous carbon, and in particular, a M-N—C catalyst as described above is provided. The method involves mixing a nitrogen/carbon source, silica sourcesof varying particle sizes, perfluorinated polymer, and optionally a metal source. Details for each of these components are set forth above. The varying particle sizes of silica impart hierarchical porosity in the final nitrogen-doped carbon which has beneficial electrocatalytic properties. In a refinement, path pis followed in which a solvent reaction/evaporationis performed followed by mechanochemical mixing (ball-milling step), and then a first pyrolysis ad a second pyrolysis as described above to produce the nitrogen-doped porous carbon. For example, the first pyrolysis being performed at a first temperature greater than about 800° C. and under an inert atmosphere (e.g., nitrogen gas as set forth above) and the second pyrolysis being performed under a reductive atmosphere (e.g., ammonia-containing as set forth above) at a second temperature that is greater than about 800° C. In another refinement, the solvent reaction/evaporation step is skipped and path pis followed in which the mechanochemical mixing stepis being proceeded to directly. In a refinement, metal source(e.g., a metal nitrate salt) can be added to produce the metallic-substituted nitrogen-doped carbon. In a refinement, the average pore diameters are from about 5 to 100 nm and the average BET surface area is from about 400 to 1000 mg.

In a variation, the number of pyrolysis steps can also be reduced to a single mechanochemically mixing step (e.g., ball-milling) followed by a single pyrolysis under an inert atmosphere as described above (e.g., N-containing) or a reductive atmosphere as described above to produce the nitrogen-doped carbon. The porosity is variable and is by the size of silica used, and that multiple pore size(s) (e.g., meso- and micropores) are achievable by the present method. In a refinement, the average pore diameters are from about 5 to 100 nm and the average BET surface area is from about 400 to 1000 mg.

Referring to, a flowchart depicting a method for treating the off-gases from the process ofis provided. The process ofproceeds through the decomposition of a perfluorinated polymer (e.g., polytetrafluoroethylene) in conjunction with a nitrogen/carbon source which similarly to the decomposition of hydrogen-containing fluoropolymers (e.g., polyvinyl fluoride) gives HF as a major product. [38, 39]. The HF reacts with SiO(silica source) to produce SiFand HO according to the following reaction where (v)=vapor phase, (s)=solid phase:SiO(s)+4HF(v)→SiF(v)+2HO(v)

The off-gas productsinand B are composed of inert gas N, unreacted HF, SiF, olefin products, trace hydrofluoropolymer fragments, trace CO/CO. The major product(s) of the decomposition reaction is expected to be a Light Key (more volatile key) consisting of SiFand unreacted HF and a Heavy Key (less volatile key) consisting of olefins (propene, dienes, etc.)(). While the current process is a far expedited way to a porous carbon with high electrocatalytic activity, it also produces SiFwhich is a toxic and corrosive gas. Advantageously, the SiFmethod can be used in existing methods for the production of polycrystalline silicon (polySi).[40] These prior art methods to utilize SiFgas as a feed to react with sodium (Na), or Magnesium (Mg).[41] This allows the avoidance of methods in which the SiFis first converted to monosilane (SiH)

The major gaseous product(s) from the current process is gaseous SiF, HF, and olefin products. These products can be separated by fractionationinto a Light Key (LK)containing HF and SiFand a Heavy Key (HK)containing majority gaseous olefin compounds. The Heavy Key (HK) may be separated and filtered according to established methods. The Light Key (LK) must pass through first a demister and HF Scrubberthat will use caustic to remove HF as a filtrate. The Light Key (LK) with HF removed will then pass to Baghouse(s)to remove entrained particulate(s) and a drierto remove all water which is necessary before sodium pellets are introduced in the final stage. The final product is a Si-fluoride product mixturethat is separated and reduced stepaccording to established methods to yield the final polycrystalline silicon material. Advantageously, the described process provides with two valuable products (porous carbon) and (polySi) in dotted boxes of.

In another embodiment, an electrochemical cell using the catalysts set forth above is provided. Referring to, electrochemical cellincludes an electrolytecontained in a vessel. Electrochemical cellalso includes a catalytically active electrodehaving an active materialdisposed on a current collector. The active material includes the catalysts and in particular, the M-N—C catalysts set forth above. Electrochemical cellalso includes a counter electrode. Characteristically, the catalytically active electrodeand the counter electrodeare immersed in the electrolyte. A voltage sourcebiases the catalytically active electrode relative to the counter electrode.

In a variation, electrochemical cellis configured to reduce of COto CO. In this regard, a system including electrochemical cellincludes a carbon dioxide source. Typically in this variation, catalytically active electrodeis a cathode that is negatively biased with respect to the counter electrode in order to drive the carbon dioxide source for introducing COinto the electrochemical cell.

In another variation, electrochemical cellis configured to reduce Nand/or nitrate.

In another variation, electrochemical cellis configured for an oxygen reduction reaction.

In still another variation, electrochemical cellis configured for an oxygen evolution reaction.